In the 1850s, Helmholtz identified the Irradiation Effect, a phenomenon in which subjects identify a white square as larger than a same-size dark square. We investigated this effect by asking participants (N=36) to make same/different size judgments of two 3-D cubes, displayed briefly (167 ms) and masked in a centered vertical arrangement against a white background. Size differences were made by increasing line segment length by 10% from small to large cubes. Cube pairings (same/different), cube size (large/small), cube shading (one of the cubes was 0%, 25%, or 50% gray), and shading position (top/bottom) were manipulated. For same size cubes, accuracy in both large and small pairs was dependent on cube shading and shading position; increased shading of the bottom cube led to diminished same judgment accuracy. For different trials, accuracy improved in conditions pairing a small unshaded top cube and a large shaded bottom cube; accuracy increased parametrically with darker shading. The shading position effect can be understood in terms of differential attentional deployment to upper and lower visual fields and in light of the lower field advantage in figure-ground assignment (Vecera, Vogel & Woodman, 2002). The pattern of costs in same judgments and benefits in different judgments as a function of increased shading may be because darker shading resulted in size overestimation, an effect contrary to Helmholtz's observations. Several additional experiments are in progress. If the relative salience of object to background is the mechanism behind overestimation, the pattern should be unchanged with a photonegative manipulation (Black background, with 100%, 75%, and 50% gray cubes). We also plan to investigate if the effect is reliant upon 2-D or 3-D representational format by using “flattened” cubes with the same line segments but that are not 3-D.